This invention relates to a method and an apparatus for separating gases, for example, removing acid gases from a synthesis gas stream containing hydrogen and derived from a partial oxidation or gasification process.
In the production of hydrogen, synthesis gas containing hydrogen as well as other undesired constituents, is derived from various processes such as steam methane reforming, the water gas shift reaction, and the gasification of various solids such as coal, coke, and heavy liquid hydrocarbons present in oil refinery waste products. The undesirable gas components include “acid gases” such as carbon dioxide and hydrogen sulfide.
As it is advantageous to “sweeten” the synthesis gas by removing the acid gases before further processing, various types of acid gas removal systems are used. Acid gas removal systems could use either chemical or physical solvents. Acid gas removal systems which use a physical solvent employ solvents such as dimethyl ethers of polyethylene glycol, methanol, or propylene carbonate, which is brought into contact with the synthesis gas under high pressure (e.g., 1,200 psia) wherein the acid gases are preferentially absorbed by the solvent. The solvent is then depressurized in a series of “flash expansions” which liberate the dissolved acid gases from the solvent. The acid gas removal system yields substantially separate gas streams for the hydrogen sulfide and the carbon dioxide. The hydrogen sulfide is directed to a sulfur recovery unit, which most often uses a Claus process to reclaim the sulfur. The carbon dioxide is normally vented to the atmosphere.
However, so as not to further contribute to global warming believed to be caused by greenhouse gases such as carbon dioxide, it is advantageous to sequester the carbon dioxide rather than release it to the atmosphere. Considering the volume of gas to be sequestered, it is preferable to use geological formations such as oil wells or underground saline aquifers to store the carbon dioxide. The carbon dioxide may be transported by pipeline and pumped into the well head or aquifer. Sequestration in oil wells confers the added benefit of enhancing oil recovery from operating wells.
Sequestration of the carbon dioxide requires that substantial compression and pumping facilities be added to the acid gas removal system in view of the high pressures and large gas volumes which sequestration entails. It is calculated that, for pipeline transport and sequestration of the gases, the carbon dioxide will need to be compressed to pressures as great as 200 bar. Of the various steps involved in the removal of carbon dioxide from a synthesis gas stream including capture of the gas, compression, and transportation to the storage site, compression can account for more than 50% of the cost of the process. It is not surprising, therefore, that efforts have been made to optimize the compression step of the process.
In a paper entitled “Shift Reactors and Physical Absorption for Low-CO2 Emission IGCCs” (Journal of Engineering for Gas Turbines and Power, April 1999, Volume 121, P. 295) authors Chiesa and Consonni describe operation of an acid gas removal system wherein the expansion ratios of the expansion stages of the solvent which liberate the dissolved carbon dioxide are constant for all expansion stages. In this paper, Chiesa and Consonni teach that the power consumption of the separation and compression section of an acid gas removal system does not change appreciably when the pressures of the expansion stages are varied (page 301).
In a subsequent paper entitled “Co-production of Hydrogen, Electricity and CO2 from Coal with Commercially Ready Technology” (International Journal of Hydrogen Energy, 30 (2005) 747-767) authors Chiesa, Consonni et al teach operating an acid gas removal system with “four flash drums to reduce CO2 compression power” and wherein “the pressures of intermediate flash drums are set to minimize overall CO2 compression power” (page 753). Table 4 (page 760) from the paper shows that between the second and third expansion stages the expansion ratio is increased. However, a constant expansion ratio is used between the third and fourth stages.
In contrast to the aforementioned teachings of the prior art, applicants have found that further significant efficiency improvements can be obtained in the operation of an acid gas removal system that includes compression of the carbon dioxide if at least three pressure reduction stages are used wherein the expansion ratios of each of the stages are increasing. In the method according to the invention, by operating with increasing expansion ratios, more of the carbon dioxide is liberated at elevated pressures, which reduces the work required later for compression. Calculations show that a decrease in compression power consumption as great as 4.5% over the aforementioned prior art (Chiesa et al, 1999) may be obtained by the method of acid gas removal according to the invention.